This invention relates to the field of lasers. More particularly, this invention relates to the field of lasers where an external cavity provides laser tuning.
In WDM (wavelength division multiplexing) optical communication, multiple wavelengths of light each carry a communication signal within a single optical fiber. Each of the multiple wavelengths of light is provided by an individual laser. In general, each wavelength is provided by an individual laser specifically designed to provide a specific wavelength. These individual lasers do not include a tuning capability. In other words, these individual lasers provide the specific wavelength for which they are designed and only the specific wavelength. This means that tens or possibly hundreds of different individually designed and fabricated lasers are required to build a WDM system that includes tens or hundreds of channels. In some WDM applications, it would be highly desirable to have a tunable laser that could be tuned to a specific wavelength. Such a tunable laser could be used as a replacement for the tens or hundreds of individually designed and fabricated lasers required to build the WDM system that includes the tens or hundreds of channels.
In U.S. Pat. No. 5,230,005, Rubino et al. teach a laser which includes a resonant cavity formed by an end mirror and a reflecting mirror. A spatial light modulator placed near the reflecting mirror allows a lasing wavelength to travel through the resonant cavity while blocking other wavelengths. A first problem associated with the laser taught by Rubino et al. is that the end mirror and the reflecting mirror provide a fixed length resonant cavity. As such, only lasing wavelengths which meet a resonant condition for the fixed length resonant cavity will lase. Thus, the laser taught by Rubino et al. is not continuously tunable. Rather it is only tunable in discrete steps. A second problem associated with the laser taught by Rubino et al. is that the spatial light modulator selectively allows the lasing wavelength to transmit through the spatial light modulator, reflect from the reflecting mirror, and return through the spatial light modulator. As the lasing wavelength transmits through the spatial light modulator, the lasing wavelength forms a focus at the plane of the spatial light modulator. On the return through the spatial light modulator, the lasing wavelength has a spot size due to the round trip between the spatial light modulator and the reflecting mirror. Accordingly, only a portion of the lasing wavelength is expected to pass through the spatial light modulator on the return through the spatial light modulator. Thus, the laser taught by Rubino et al. is expected to be inefficient due to the loss of light on the return through the spatial light modulator.
In an article entitled, xe2x80x9cSpectrally narrow pulsed dye laser without beam expander,xe2x80x9d in Applied Optics, Vol. 17, No. 14, Jul. 15. 1978, Littman and Metcalf teach an external cavity laser in what has come to be known as a Littman-Metcalf external cavity laser. The Littman-Metcalf external cavity laser includes a partially reflecting mirror, a dye lasing medium, a diffraction grating, and a rotatable mirror. The partially reflecting mirror and the rotatable mirror form a resonant cavity. The dye lasing medium outputs multiple light wavelengths which are reflected by the diffraction grating to the rotatable mirror. By rotating the rotatable mirror to return a particular light wavelength to the dye lasing medium, a lasing wavelength is chosen.
In U.S. Pat. No. 5,319,668, Luecke teaches a modified Littman-Metcalf external cavity laser where the dye lasing medium is replaced by a laser diode and where the pivot point for the rotatable mirror is selected to provide a continuous tuning capability. However, because the modified Littman-Metcalf external cavity laser taught by Luecke is accomplished by mechanically rotating a mirror, the modified Littman-Metcalf external cavity laser provides a slow tuning speed. It is believed that for the modified Littman-Metcalf external cavity laser taught by Luecke, the tuning speed is on the order of 1 sec.
It is believed that there are MEMS based Littman-Metcalf external cavity lasers currently under development that include the rotatable mirror originally taught by Littman and Metcalf. However, it is believed that these MEMS based Littman-Metcalf external cavity lasers, while providing a faster tuning speed than 1 sec., do not provide an adequate tuning speed for WDM applications. It is believed that tuning speeds for the MEMS based Littman-Metcalf external cavity lasers are on an order of 10 xcexcsec.
What is needed is a continuously tunable laser source, which is efficient, which provides a fast tuning speed, and which is economical.
An external cavity laser of the present invention comprises a laser source, a collimation optical element, a blazed diffraction grating, a transform optical element, and a light modulator. The laser source is operable to produce a light output comprising a range of light wavelengths. The collimation optical element couples the laser source to the blazed diffraction grating. The collimation optical element collimates the light output. The blazed diffraction grating diffracts the light output into a first diffraction order. The transform optical element couples the blazed diffraction grating to the light modulator, which is located in a transform plane of the transform optical element. The transform optical element converts the first diffraction order to position in the transform plane by focusing the range of light wavelengths to the transform plane.
The light modulator comprises an array of light modulating pixels selectively operable in first and second modes. A particular light modulating pixel in the first mode reflects light along a return path. The particular light modulating pixel in the second mode directs light away from the return path. The first mode is selectively operable to adjust a return path length by at least a half wavelength. In operation at least one of the light modulating pixels operates in the first mode returning a lasing wavelength along the return path and creating a resonant cavity for the lasing wavelength while operating a remainder of the light modulating pixels in the second mode to direct a remainder of the range of light wavelengths away from the return path.